The mechanical properties introduced to the steel through this processing route are virtually equivalent to those obtained by heat treating conventionally rolled or forged steel. TMCP involves controlled hot working and microalloyed steel compositions.
When TMCP is chosen as the process route, the rough steel section (i.e. the slab) is heated to a temperature regularly used for hot working operations (about 1200°C). The initial hot working ('roughing') is carried out in a normal fashion, but the final hot work reduction or 'finishing pass' is carried out at a lower temperature than would be used for older processes. Plastic deformation at this lower temperature promotes fine grain sizes and retards precipitation. The final hot working may continue down to temperatures below the Ar3 critical temperature (transformation from austenite to ferrite). This requires heavy rolling equipment capable of deforming the steel at low hot working temperatures. The optimum precipitate size and dispersion is obtained when the finish rolling temperature is around 775°C
The cooling which follows brings the steel to the transformation temperature range, and the austenite to ferrite transformation results in fine ferrite grains and fine dispersed precipitates. For some TMCP steels, this last stage of cooling, during which transformation is completed, is accelerated by water cooling, to give a finer grain size. Accelerated cooling can sometimes result in bainite formation as well as, or instead of, ferrite formation.
There are numerous methods of TMCP, some of which are illustrated against conventional processes in Fig.1. The TMCP routes broadly fall into three main categories:
- Controlled rolled down from the normalising temperature, yet fully austenitic (above the Ar3 temperature) followed by a rapid cool of approximately 10°C/sec. The aim of this process is to refine the grain size by controlled rolling and to increase the strength by suppressing the formation of ferrite and pearlite in favour of a strong tough bainite.
- Controlled rolled both above the Ar3 and below that temperature, in the austenite ferrite mixed region. In addition to austenite grain refinement, the recrystallised grains are flattened and nucleation of fine ferrite is encouraged by the deformation. At a temperature above the Ar1, the controlled rolling is interrupted, and followed by rapid cooling to room temperature or an intermediate temperature.
- Controlled rolling is performed as part of a preliminary processing, followed by cooling and reheating to just above the Ar3 temperature and then rapid cooling to well below the Ar1 temperature. The purpose of this technique is to develop the finest equiaxed austenite grain size before the controlled cooling begins.
By appropriate choice of deformation temperature and strain rate, the strength of a steel can be increased. The strength of a TMCP steel is higher than for normalised steel of the same composition. Thus a TMCP steel has a leaner composition (lower alloy content) than a conventional normalised steel of the same strength.
The lean composition of TMCP steels can be beneficial with respect to weldability. This is detailed further in Are TMCP (Thermo-Mechanically Controlled Processing) steels readily weldable?
References
1. Yajima et al.,'Extensive Application of TMCP-manufactured High Tensile Steel Plates to Ship Hulls and Offshore Structures' Mitsubishi Heavy Industries Technical Review Vol 24 No. 1, February 1987
2. Nishioka K: 'Market requirements of thermomechanically processed steel for the 21st century'. Steel World 2000 5 (1) 61-67
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